1. Field of the Invention
The invention relates to a threaded tubular connection for a tubular string which is subjected to dynamic bending loads, comprising a male tubular element provided with a male threaded portion and a female tubular element provided with a female threaded portion.
2. Discussion of the Background
That type of threaded connection is intended for making strings for hydrocarbon or the like wells.
In addition to relatively constant (static) axial tensile loads, under the action of waves, the wind, the tides and sea currents, strings connecting an offshore platform to the sea bed are subjected to variable (dynamic) bending loads. Said loads are transmitted from one tube to the next in the string through the threaded connections.
Drill string components for rotary drilling, in particular drill pipe, heavy weight drill pipe and drill collars, are also subjected to variable bending loads due to rotation of the string when drilling a non-vertical (deviated) borehole and the threaded connections between such components also have to bear and transmit such variable bending loads.
FIG. 3 shows that the last threads of the male element and the female element are subjected to a maximum bending moment which has to be transmitted in addition to the whole of the axial tensile load.
This results in dynamic tensile loads known as repeated loads at the root of the last threads, and more particularly the last male threads, which initiates fatigue cracking at that location leading to catastrophic rupture of the string.
International patent applications WO-A-01/75345 and WO-A-01/75346 describe solutions for reducing stresses at the roots of said threads, but the resulting improvement may prove insufficient.
Said dynamic loads also cause friction between the parts of the male and female elements in contact, leading to cracking due to fretting fatigue.
When one of the tubular elements has an axial abutment (or axial stop) surface at its free end which abuts against the other tubular element, the abutment that occurs enables to absorb part of the bending moment. However; to form such abutment surfaces, it is necessary to choose thicker tubes or to increase their thickness locally by upsetting, which introduces increased costs.
In the case of threaded tubular connections having so-called wedge or self-locking threads such as disclosed in documents U.S. RE30647, U.S. RE34467, WO2004/106797, the content of which documents is incorporated here by reference, there is no such abutment surface because the pin and box thread stabbing flanks come in contact and the pin and box thread load flanks come in contact during make-up and stop further threading of the connection, fulfilling then the function of abutment surfaces. Separate abutment surfaces are not only unnecessary on such threads, in addition they are undesired as it is difficult to synchronize end of make-up by abutment surfaces and by self-locking threads.